LONG-TERM SOMATOTROPIC AND GALACTOPOIETIC EFFECTS OF A (1-30)ETHYL AMIDE ANALOG OF GROWTH HORMONE-RELEASING FACTOR

Thirty-five lactating Holstein cows (165 +/- 8 DIM) received the follo wing treatments for 96 d: uninfused controls; i.v. infusion of .9, 2.7 , or 8.1 mg/d of Ile(2), Ser(8,28), Ala(15), Leu(27), Hse(30)-bovine g rowth hormone-releasing factor (1-30) ethyl amide; or i.v. infusion of 12 mg/d of recombinant Leu(27), Hse(45)-bovine growth hormone-releasi ng factor (1-45) lactone. Concentrations of somatotropin in serum and SCM yield of cows infused with 1-30 releasing factor increased quadrat ically as the dose increased. Responses for somatotropin and SCM yield were quantitatively similar for cows infused with 2.7 and 8.1 mg of 1 -30 or 12 mg of 1-45. While concurrently infused with the 1-30 or 1-45 as described, cows retained their ability to release somatotropin fol lowing an acute i.v. injection of 10 nmol/100 kg of BW of either growt h hormone-releasing factor 1-30 or 1-45. Mean concentrations of IGF-I in serum increased similarly in magnitude for cows infused at all dose s of 1-30 or 1-45. The 1-30 releasing factor generally increased IGF-b inding protein-3, but had little effect on IGF-binding protein-2. The 1-45 releasing factor did not significantly affect either binding prot ein. Yield of SCM was correlated with serum concentrations of somatotr opin, but not with IGF-I. Concentrations of NEFA in serum were elevate d through 36 and 50 d in response to the highest doses of 1-30 and 1-4 5, respectively. Treatment did not affect DMI, BW, or body condition s core. In conclusion, continuous i.v. infusion of 2.7 and 8.1 mg/d of 1 -30 and 12 mg/d of 1-45 similarly increased yield of SCM, somatotropin , and IGF-I without inducing refractoriness in the ability of the ante rior pituitary gland to release somatotropin in response to a concurre nt, acute challenge with 1-30 or 1-45.